There are many different physical phenomena that involve both sound and light. In photoacoustics, sounds are created by very short pulses of laser light impinging on materials. Through holographic interferometry and fiber optic sensors, light can be used to measure and visualize sound. Sound can also have an influence on light, for example when an acousto-optic modulator (AOM) is used to shift the frequency of light. However, perhaps the most interesting and most baffling interaction between sound and light is the phenomenon known as sonoluminescence.
Sonoluminescence is the process in which intense sound waves are used to induce cavitation in tiny bubbles trapped within a degassed liquid medium. Under just the right conditions, the temperature of the gas inside the cavitating bubble reaches tens of thousands of degrees and causes the bubble to become a tiny point of light. My experiments were done in a standard apparatus which has two piezoelectric transducers (PZTs) attached on diametrically opposed faces of a 100 mL spherical flask filled with filtered de-ionized water. The PZTs are driven by a frequency generator connected to an amplifier. The water was degassed by pumping for about 15 minutes with a liquid-nitrogen-cooled absorption pump. Another smaller PZT connected to the bottom of the flask acts as a microphone to monitor the response of the flask and sound of the collapsing bubbles. After hours of careful adjustments in near darkness, I was able to achieve sonoluminescence in room temperature degassed water at resonant frequencies between 26.640 kHz and 26.669 kHz. The microphone signal was between 789 and 860 mV-rms. This sonoluminescence proved to be rather unstable and dim; when I later experimented with colder water (about 10 C), the light was brighter and more stable. The sonoluminescence in the colder water took place at frequencies between 26.164 kHz and 26.634 kHz and pick-up amplitudes between 434 mV-rms and 560 mV-rms.
For the future, I plan to study the intensity of the light using a photomultiplier, a type of very sensitive light detector, with the goal of gaining quantitative information on the effect of conditions like temperature and gas content on the brightness of the "star in a jar."
This research was supported by the Simons Foundation.

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